1. Field of the Invention
The present invention relates generally to well cement compositions and methods, and more particularly, to such compositions and methods which are suitable for cementing high temperature wells containing carbon dioxide.
2. Description of the Prior Art
In the completion of high temperature subterranean wells containing carbon dioxide, e.g., geothermal wells, the use of conventional hydraulic cement compositions often results in early well failure. Because of the high static well bore temperatures involved coupled with the presence of brines containing carbon dioxide, conventional hydraulic well cements rapidly deteriorate due to alkali carbonation, especially sodium carbonate induced carbonation. In geothermal wells, which typically involve very high temperatures, pressures and carbon dioxide concentrations, conventional well cement failures have occurred in less than five years causing the collapse of the well casing.
It has heretofore been discovered that a cement material known as calcium phosphate cement formed by an acid-base reaction between calcium aluminate and a phosphate-containing solution has high strength, low permeability and excellent carbon dioxide resistance when cured in hydrothermal environments. However, calcium phosphate cement has a relatively high density, e.g., a density in the range of from about 15 to about 17 pounds per gallon, which is too high for geothermal applications. That is, in geothermal wells the hydrostatic pressure exerted by the high density calcium phosphate cement often exceeds the fracture gradients of subterranean zones penetrated by the well bore which causes the formation of fractures into which the cement is lost. While calcium phosphate cements have been developed which include hollow microspheres and as a result have densities of about 10 pounds per gallon, such light weight compositions are relatively expensive and the presence of the microspheres in the cured cement reduces its compressive strength.
Thus, there is a need for improved less expensive well cement compositions useful in cementing high temperature wells containing carbon dioxide.
The present invention provides improved cement compositions and methods which meet the needs described above and overcome the deficiencies of the prior art. The compositions are particularly useful in high temperature wells containing carbon dioxide such as geothermal wells. A composition of the present invention is basically comprised of calcium aluminate, fly ash and sufficient water to form a pumpable slurry.
Another composition of this invention is comprised of calcium aluminate, fly ash, sufficient water to form a pumpable slurry, a foaming agent, a foam stabilizer and a gas sufficient to form a foam having a density in the range of from about 9.5 to about 14 pounds per gallon.
Yet another composition of this invention is comprised of calcium aluminate, sodium polyphosphate, fly ash, sufficient water to form a pumpable slurry, a foaming agent, a foam stabilizer and a gas present in an amount sufficient to form a foam having a density in the range of from about 9.5 to about 14 pounds per gallon.
The methods of the present invention for cementing a high temperature subterranean zone containing carbon dioxide penetrated by a well bore basically comprise the steps of forming a well cement composition of this invention, pumping the cement composition into the subterranean zone by way of the well bore and allowing the cement composition to set into a hard impermeable mass therein.
It is, therefore, a general object of the present invention to provide high temperature well cement compositions and methods.
A further object of the present invention is the provision of improved carbonation resistant well cement compositions and methods.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.
As mentioned above, high temperature wells containing carbon dioxide such as geothermal wells generally require the use of well cement compositions which do not deteriorate in the presence of carbon dioxide containing brines. The term xe2x80x9chigh temperaturexe2x80x9d is used herein to mean wells wherein the static bottom hole temperature is above about 300xc2x0 F. up to as high as about 700xc2x0 F. When conventional hydraulic cements are utilized in such wells, carbonation causes dissolution of the cement which is converted into water-soluble salts. Further, severe corrosion of steel pipe takes place thereby resulting in the total disruption of the conventional cement supported well structure.
When conventional normal density cement slurries are utilized in geothermal and other similar wells, loss of circulation problems are often encountered. This is due to the weak unconsolidated formations in the wells having very low fracture gradients. When a relatively high density cement slurry is pumped into such a well, the hydrostatic pressure exerted on the weak unconsolidated subterranean zones therein causes the zones to fracture. This in turn causes the cement slurry being pumped to enter the fractures and lost circulation problems to occur. To avoid such problems, the cement compositions utilized in geothermal and other similar wells must be of light weight, i.e., have densities in the range of from about 9.5 to about 14 pounds per gallon.
By the present invention, improved well cement compositions are provided which resist high temperature carbonation deterioration. A cement composition of this invention which can be non-foamed or foamed is basically comprised of calcium aluminate, fly ash and sufficient water to form a pumpable slurry. When foamed, the cement composition includes a foaming agent, a foam stabilizer and a gas present in an amount sufficient to form a foam having a density in the range of from about 9.5 to about 14 pounds per gallon.
Another composition of this invention is comprised of calcium aluminate, sodium polyphosphate, fly ash, a foaming agent, a foam stabilizer and a gas present in an amount sufficient to form a foam having a density in the range of from about 9.5 to about 14 pounds per gallon.
The calcium aluminate can be any commercial grade calcium aluminate suitable for use as a cement. A suitable such calcium aluminate is commercially available from the Lehigh Portland Cement Company of Allentown, Pa., under the trade designation xe2x80x9cREFCON(trademark).xe2x80x9d The calcium aluminate is generally included in the cement composition in an amount in the range of from about 15% to about 45% by weight of the composition.
When used, the sodium polyphosphate includes sodium metaphosphate and sodium triphosphate as well as vitreous sodium phosphates. A suitable sodium polyphosphate for use in accordance with the present invention is commercially available from Calgon Corporation of Pittsburgh, Pa. The sodium polyphosphate can be included in the cement composition in an amount in the range of from about 5% to about 20% by weight of the composition. When included, the sodium polyphosphate combines with the calcium aluminate to form calcium phosphate in the form of hydroxyapatite.
Fly ash is the finely divided residue that results from the combustion of ground or powdered coal and is carried by the flue gases generated. A particular fly ash that is suitable in accordance with the present invention is a fine particle size ASTM class F fly ash having a Blaine fineness of about 10,585 square centimeters per gram which is commercially available from LaFarge Corporation of Michigan under the trade designation xe2x80x9cPOZMIX(trademark).xe2x80x9d Another fly ash that is suitable is an ASTM class F fly ash which is commercially available from Halliburton Energy Services of Dallas, Tex. under the trade designation xe2x80x9cPOZMIX(trademark)A.xe2x80x9d The fly ash is generally included in the composition in an amount in the range of from about 25% to about 45% by weight of the composition.
The major crystalline phase of ASTM class F fly ash is mullite (3Al2O3xc2x72SiO2). It reacts with calcium aluminate to form calcium alumino silicate (CaOxc2x7Al2O3xc2x72SiO2). Also, iron and quartz in the fly ash react with the calcium aluminate to form andradite (Ca3Fe2SiO4)3. These reactions increase the compressive strength of the set cement as compared to set calcium aluminate cement alone.
The water utilized can be from any source provided it does not contain an excess of compounds that adversely affect other compounds in the cement composition. For example, the water can be fresh water or saltwater. Generally, the water is present in the cement composition in an amount sufficient to form a pumpable slurry, i.e., an amount in the range of from about 10% to about 60% by weight of the composition.
In order to facilitate the foaming of the cement composition, a foaming agent is included in the composition. A particularly suitable and preferred such foaming agent is an alpha-olefinic sulfonate having the formula
H (CH2)nxe2x80x94CHxe2x95x90CHxe2x80x94(CH2)mSO3Na
wherein n and m are individually integers in the range of from about 6 to about 16. The foaming agent is generally included in the cement composition in an amount in the range of from about 1% to about 2% by weight of the water in the composition. The most preferred foaming agent of this type is an alpha-olefinic sulfonate having the above formula wherein n and m are each 16, i.e., a sulfonic acid C16-16 alkane sodium salt.
A foam stabilizer is also included in the cement composition to enhance the stability of the composition after it is foamed. A particularly suitable and preferred stabilizing agent is an amidopropylbetaine having the formula
Rxe2x80x94CONHCH2CH2N+(CH3)2CH2CO2xe2x88x92
wherein R is a radical selected from the group of decyl, cetyl, oleyl, lauryl and cocoyl. The foam stabilizer is generally included in the cement composition in an amount in the range of from about 0.5% to about 1% by weight of the water in the composition. The most preferred foam stabilizer of this type is cocoylamidopropylbetaine.
The gas utilized to foam the composition can be air or nitrogen, with nitrogen being the most preferred. The amount of gas present in the cement composition is that amount which is sufficient to form a foam having a density in the range of from about 9.5 to 14 pounds per gallon, most preferably 12 pounds per gallon.
In order to provide resiliency to the set cement composition of this invention, the composition may optionally include inert ground rubber particles. Such particles are produced from worn out tires and are commercially available from Four D Corporation of Duncan, Okla.
At static well bore temperatures above about 125xc2x0 F., a set retarder is required. The set retarder functions to lengthen the time in which the cement composition starts to thicken and set so that the composition can be pumped into the well bore and into the zone to be cemented before such thickening takes place. Preferred such set retarders for use in accordance with this invention are gluconic acid and citric acid. When used, the set retarder is included in the cement composition in an amount in the range of from about 0.5% to about 2% by weight of the composition.
A preferred composition of the present invention is comprised of calcium aluminate present in an amount of about 30% by weight of the composition, ASTM class F fly ash present in an amount of about 50% by weight of the composition and water present in an amount sufficient to form a slurry.
Another preferred composition of the present invention is comprised of calcium aluminate present in an amount of about 30% by weight of the composition, ASTM class F Fly Ash present in an amount of about 50% by weight of the composition, sufficient water to form a pumpable slurry, a foaming agent comprised of a sulfonic acid C16-16 alkane sodium salt present in an amount of about 1.5% by weight of the water in the composition, a foam stabilizer comprising cocoylamidopropyl- betaine present in an amount of about 0.75% by weight of the water in the composition and a gas present in an amount sufficient to form a foam having a density in the range of from about 9.5 to about 14 pounds per gallon.
Yet another preferred composition of this invention is comprised of calcium aluminate present in an amount of about 28% by weight of the composition, sodium polyphosphate present in an amount of about 19% by weight of the composition, ASTM class F fly ash present in an amount of about 49% by weight of the composition, sufficient water to form a pumpable slurry, a foaming agent comprised of a sulfonic acid C16-16 alkane sodium salt present in an amount of about 2% by weight of the water in the composition, a foam stabilizer comprising cocylamideopropylbetaine present in an amount of about 1% by weight of the water in the composition and a gas present in an amount sufficient to form a foam having a density in the range of from about 9.5 to about 14 pounds per gallon.
As previously mentioned, the above described cement compositions can include ground rubber particles present in an amount in the range of from about 10% to about 40% by weight of the compositions to improve the resiliency of the compositions. Further, when the static well bore temperature is above about 125xc2x0 F., a set retarder selected from the group of gluconic acid and citric acid is included in the cement compositions in an amount of about 1.0% by weight of the compositions.
The cement compositions of this invention may be prepared in accordance with any of the mixing techniques utilized in the art. In one preferred method, a quantity of water is introduced into a cement blender followed by the sodium polyphosphate (if used), calcium aluminate and fly ash. The mixture is agitated for a sufficient period of time to form a pumpable non-foamed slurry.
When the cement slurry formed as above is foamed, the slurry is pumped to the well bore and the foaming agent and foam stabilizer followed by the gas utilized are injected into the slurry on the fly. As the slurry and gas flow through the well bore to the location where the resulting foamed cement composition is to be placed, the cement composition is foamed and stabilized. Other liquid additives utilized, if any, are added to the water prior to when the other components of the cement composition are mixed therewith and other dry solids, if any, are added to the water and cement prior to mixing.
The methods of this invention of cementing a high temperature subterranean zone containing carbon dioxide penetrated by a well bore are basically comprised of the steps of forming a foamed cement composition of this invention, pumping the foamed cement composition into the subterranean zone to be cemented by way of the well bore and then allowing the foamed cement composition to set into a hard impermeable mass therein.